1. Field of the Invention
The present invention relates to an image transmission/reception/recording apparatus for transmitting and receiving image data, such as a facsimile apparatus, and also to an image transmission/reception/recording method.
2. Related Background Art
Recent progress in the image recording apparatus and communication technology has stimulated rapid popularization of image transmit/receiving apparatus for transmission and reception of an image, such as a facsimile apparatus, with continuously increasing requirements for higher image quality. Consequently there is longed for an apparatus capable of providing excellent tonal rendition and effecting high-speed communication. Based on such background, there are gradually becoming popular apparatus which, in the transmission, convert a multi-value signal into a binary signal for transmission by pseudo tonal rendition such as dither method or error diffusion method and, in the reception, record the received signal by processing in the binary state. In the image recording unit of the facsimile apparatus, digital image recording is generally employed with a recording head of thermal transfer type or ink jet type. In such recording, a multiple head in which plural image recording elements are integrated is generally employed for increasing the recording speed. For example, for the ink jet recording head, there is generally employed a multi nozzle head in which plural nozzles are integrated, and, in the thermal head for thermal transfer recording, plural heater elements are usually integrated.
FIG. 1 is a block diagram of a conventional facsimile apparatus employing such multiple head.
Facsimile apparatus A, B are composed of image reader units 21a, 21b, image processor units 22a, 22b for example for tone correction, binarizing units 23a, 23b for binary digitization of tone-corrected image signal; image recorder units 24a, 24b for image recording with binarized image signal, and interfaces 25a, 25b.
In the image transmission from the facsimile A to B, the image signal subjected to tonal correction in the image processor unit 22a and binary digitized in the binarizing unit 23a is supplied, through the interfaces 25a, 25b, to the image recorder unit 24b of the facsimile B and recorded therein.
In such multiple head, however, the image recording elements are difficult to prepare in uniform manner and inevitably involve certain fluctuation in the characteristics. For example, the multiple ink jet recording head involves fluctuation in the shape of nozzles, and the multiple thermal transfer recording head involves fluctuation in the shape or resistance of heater elements. The fluctuation in the characteristics among the image recording elements leads to uneven size or density of the dots recorded by the image recording elements, eventually resulting in an unevenness in the density of the recorded image. The quality of the received image has been significantly deteriorated by such unevenness. Particularly in color image communication, the deterioration of image quality is significant due to unevenness in color or error in color reproduction.
In order to overcome such drawback, there have been proposed various methods of obtaining a uniform image by correcting the signals given to the image recording elements. In one of such methods shown in FIGS. 2A to 2E, if a multiple head 1 with an array of recording elements 2 shown in FIG. 2A receives uniform input signals FIG. 2B and provides a density unevenness shown in FIG. 2C, the input signals are corrected as shown in FIG. 2D to a higher level for the recording elements giving a lower density and a lower level for the recording elements giving a higher density. In case of a recording method capable of modulating the dot diameter or dot density, the dot diameter to be recorded by each recording element may be modulated according to the input signal. For example, in the ink jet recording method of piezoelectric type, the driving voltage or the pulse duration for each piezoelectric element is varied according to the input signal, and, in the thermal transfer recording, the driving voltage or pulse duration to each heater element is likewise varied, whereby the dot diameters or densities of different recording elements are made uniform as shown in FIG. 2E. If the variation in the dot diameter or density is impossible or difficult, the number of dots is so regulated, according to the input signals, as to provide a larger number of dots in the recording elements providing a lower density and a smaller number of dots in the elements provide a higher density, thereby providing a uniform density as shown in FIG. 2E.
The amount of correction is determined in the following manner, as an example, in case of a multiple recording head with 256 nozzles.
Let us consider a case in which uniform image signals S provide a density distribution shown in FIG. 3. There are determined at first the average density OD for this head, then the densities OD.sub.1 -OD.sub.256 corresponding to respective nozzles, and the differences .DELTA.OD.sub.n =OD-OD.sub.n (n=1-256). If the tonal characteristic, namely the relationship between the image signal and the recorded density, is given by a chart shown in FIG. 4, a density correction by .DELTA.OD.sub.n requires a signal correction .DELTA.S. This can be achieved by a table conversion as shown in FIG. 5. In FIG. 5, a line A has an inclination of 1.0, whereby the input signal is released without any conversion. On the other hand, a line B has an inclination (S-.DELTA.S)/S, whereby an input S provides an output S-.DELTA.S.
Consequently a density OD can be obtained with the n-th nozzle by applying a table conversion as indicated by the line B in FIG. 5 to the image signal corresponding to said n-th nozzle. The unevenness in density can be corrected and a uniform image can be obtained by applying such process to all the nozzles. Stated differently, correction of unevenness in density is rendered possible by determining, in advance, the table conversions to be applied to the image signals, respectively corresponding to different nozzles.
FIG. 6 is a block diagram of a facsimile apparatus in which such unevenness correction is adopted. The facsimile apparatus A, B are provided with unevenness correcting ROM's 26a, 26b and correction data RAM's 27a, 27b, in addition to the aforementioned image reader units 21a, 21b, image processor units 22a, 22b for tonal correction etc., binarizing units 23a, 23b, image recorder units 24a, 24b and interfaces 25a, 25b. The unevenness correction ROM's 26a, 26b store plural conversion tables as shown in FIG. 5, and effect correction of unevenness by selecting suitable conversion tables according to signals from correction data RAM's 27a, 27b.
In case of image transmission from the facsimile apparatus A to B, the signal processed in the image processor unit 22a is sent to the unevenness correction ROM 26b of the facsimile B. Said ROM 26b stores a plurality of tables for correcting the density unevenness of the image recorder unit 24b of the facsimile B, and effects the unevenness correction on the input signal, according to a table selected by the correction data RAM 27b.
A received image free from unevenness can be obtained in this manner, but this method requires transmission and reception of multi-value image data since the table conversion is conducted in the unevenness correction ROM 26b. Since the image data contain a large amount of information, such transmission and reception of multi-value data require enormous time and cost.
This drawback becomes even more conspicuous in a color facsimile apparatus for transmitting a color image, since multi-value data corresponding to plural recording heads have to be transmitted.
In the foregoing there has been explained the fluctuation among the image recording elements in a recording head, but same applies to the fluctuation among different recording heads. It is difficult to produce such recording heads without any fluctuation in performance, and the recording operation with such heads without any correction will result in a fluctuation in the image density, because of the fluctuation in the head performance. Thus there will result a drawback that an image appears dark in a facsimile apparatus but light in another.
FIG. 7 shows a facsimile apparatus conceived capable of copying with such drawback.
Image signals read in image reader units 21a, 21b are subjected to image processing, such as logarithmic conversion, in image processor units 22a, 22b. Each of density corrector units 33a, 33b has a look-up table representing 41 straight lines of which inclination varies by 0.01 within a range from 0.8 to 1.2, and selects a line corresponding to the density obtained by the recording head of the image recorder unit. For example the line A in FIG. 5 is a standard line with an inclination 1.0, and the line B of smaller inclination is selected if the recording head provides a higher density. Thus, in response to the input signal S, there is obtained a corrected image signal S-.DELTA.S, which is reduced by a factor (S-.DELTA.S)/S.
In case of image transmission from the facsimile apparatus A to B, the output signal of the image processor unit 22a of the facsimile A is transmitted to the density corrector unit 33b of the facsimile B, whereby a density correction is conducted corresponding to the recording head thereof to provide an image of an appropriate density in the facsimile apparatus B.
Also in this conventional structure, however, transmission and reception of multi-value image data are indispensable because the correction by table conversion is conducted at the receiving side, involving significant time and cost for such transmission and reception of image data.